After the occurrence of the 2011 magnitude 9 Tohoku earthquake, seismicity in the overriding plate changed. This chapter considers some possible evidence of the influence of crustal fluid/water on the occurrence of the triggered seismicity after the 2011 Tohoku-Oki earthquake. It estimates the permeability from the observed hypocenter diffusion. Shallow seismic activity and crustal deformation are strongly affected by the water dehydrated and upwelling from the subducting Pacific plate. Spatiotemporal expansion of hypocenter areas of some earthquake swarms can be attributed to fluid diffusion. In the upper crust, the earthquakes seem to be distributed in seismic high-velocity areas rather than in seismic low-velocity areas. In the lower crust, the seismic low-velocity areas appear to be elongated along N-S or NE-SW, the strike of the island arc. The chapter also estimates three-dimensional seismic velocity structure using the double-difference tomography method.
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